subroutine get_avhrr_ctp(debug, line, elem, along_track_idx, across_track_idx,
     *                         avhrr_ctp, mean_avhrr_ctp)

c-----------------------------------------------------------------------------------
c
c Description: Calculate mean of all collocated AVHRR cloud top pressure values   
c              in the current HIRS IFOV.                   
c
c Input parameters:
c debug                     Debug write flag
c line                      HIRS line number (1-1100)
c elem                      HIRS pixel number (1-56)
c along_track_idx           AVHRR scan line numbers of collocated pixels
c across_track_idx          AVHRR element numbers of collocated pixels
c avhrr_ctp                 AVHRR cloud top pressures from PATMOS-x data
c
c Output parameters:
c mean_avhrr_cpt            Mean of collocated AVHRR cloud top pressures
c
c Revision history:         03/14       R. Frey       Original version
c
c Calls                     None
c
c----------------------------------------------------------------------------------

      implicit none
      save

c----------------------------------------------------------------------------------

c     Calling arguments.
      real*4  avhrr_ctp(409, 15000), mean_avhrr_ctp
      integer across_track_idx(100,56,1100), along_track_idx(100,56,1100)
      integer debug, line, elem

c     Local variables.
      real ctpsum
      integer ntot
      integer i, j, k

c-----------------------------------------------------------------------------------

c     Get land cover indecis from collocated PATMOS-X data.

      ntot = 0
      ctpsum = 0.0

      do i = 1, 100

        j = along_track_idx(i,elem,line)
        k = across_track_idx(i,elem,line)

        if(j .ne. 0 .and. k .ne. 0) then
          if(avhrr_ctp(k,j) .gt. 0.0) then

            ctpsum = ctpsum + avhrr_ctp(k,j)
            ntot = ntot + 1

            if(debug .gt. 1) then
              write(*,'(4i6,2f12.3)') i, j, k, ntot, avhrr_ctp(k,j), ctpsum
            end if

          end if
        end if

c----------------------------------------------------------------------------------

      enddo

c-----------------------------------------------------------------------------------

c     Calculate mean cloud top pressure.
 
      if(ntot .gt. 0.0) then
        mean_avhrr_ctp = ctpsum / ntot
      else
        mean_avhrr_ctp = -99.99
      end if

c-----------------------------------------------------------------------------------

      if(debug .gt. 0 .or. mean_avhrr_ctp .eq. -99.99) then
        write(*,'(''Collocated AVHRR CTP at line '', i5, '' and element '',
     *            i5, '': '', f8.3)') line, elem, mean_avhrr_ctp
      end if

c-----------------------------------------------------------------------------------

      return
      end